Protector for electric circuits



INVENTOR. EUSSMAA/A/ Q2 AM TTORNEY.

March 4, 1941- F. H. BUSSMANN PROTECTOR FOR ELECTRIC CIRCUITS Filed April 25, 1958 Patented Mar. 4, 1941 UNITED STATES PATENT OFFICE 3 Claims.

My invention relates to an improvement in protectors for electric circuits, such as plug fuses, and similar devices containing a fuse link and adapted for use in plug fuse holders, and more particularly relates to the sealing against venting of such devices having fuse links of relatively high melting points.

An object of my invention is to provide a plug fuse having a link of relatively high melting point and capable of operating successfully under all conditions of overload, including heavy short circuit.

A further object of my invention is to provide a plug fuse having a fuse link enclosed in a substantially gas tight casing to prevent the exit of hot vapors therefrom and to take full advantage of the pressure generated on blowing the link to quickly extinguish the are then formed.

A still further object of my invention is to accomplish the'above mentioned objects without adding appreciably to the cost of manufacturing the device.

When a fuse link is blown on overload more or less of it is volatilized, the amount being dependent on the design of the link, the degree of overload and other circuit conditions. The current will be carried by an are at least momentarily with consequent volatilization of more metal. If the link be in an enclosure the resulting pressure may be so great as to disrupt the enclosure if the are be not very quickly extinguished. As increased pressure tends to extinguish the are a pressure safely within the power of the enclosure to withstand is an advantage.

In the past most plug fuses have been made with links of metals having relatively low melting points, such as lead, or more commonly, zinc. As those metals are not only of relatively low melting point but are also relatively poor electrical conductors the quantity of metal therein must be relatively large. This results in a relatively large amount of metal being volatilized on short circuit with consequent heavy pressures.

It is customary to prevent this pressure from attaining a disruptive value by venting the enclosure, that is by allowing a portion of the vapors to escape from the enclosure. Various methods have been used to control the escaping Vapors as regards both the quantity and the temperature thereof. The vapors as generated are of high temperature and if allowed to escape freely and without a substantial reduction in tempera.- ture could ignite combustible material which might be nearby. If the fuse link be made of a higher melting point material than commonly used it will be smaller for the same rating. If made of such higher melting point material as silver or copper the greater electrical conductivities of these metals will make for still greater reduction in the amount of metal required. This means. less metal volatilized when the link is blown.

The resulting pressure within the enclosure depends. directly on the amount of metal volatrilized and the temperature'of the vapor. As the temperature of the arc is always greatly in excess of the melting point of the link it is possible fora link of high melting point material to cause less pressure than one of lower melting point because of the smaller amount of metal volatilized.

Fuse links of silver have been quite extensively used in Europe. in fuses somewhat similar to the plug fuses used in this country, for example the German patronen type of fuse. Due to differences in operating conditions in this country such fuses have not been used to any extent here.

I have discovered that if I use a good electrical conductor having a relatively high melting point, such as copper or silver, for my fuse link, certain advantages are sometimes obtained and the pressures generated, even on short circuit, are such that I can make my enclosure gas-tight without having to make it unusually strong. By my invention I make the enclosure gas'tight by the use of a sealingwasher, the nature of which will be hereinafter described.

When the fuse enclosure can be made gas-tight without danger of disruptive pressure it is possible to take full advantage of the effect of pressure in quickly extinguishing the arc. The more or less expensive and complicated arrangements required to control the amount of venting and to reduce the temperature of the escaping gases below the possible danger point become unnecessary when no venting is permitted.

Sometimes for reasons other than the pressures attained on short circuit, it is desirable to use a fuse link of relatively good electrical conductivity. My invention is of particular advantage in such cases. Such metals usually have higher melting points and higher latent heats of fusion and of vaporization than the metals commonly used. Because of these properties the cooling of such vapors as may be allowed to escape is made more diflicult and uncertain than if the link were made of one of the more commonly used metals. The prevention of the escape of any of the hot vapors is, then, especially desirable when the link is made of one of the better conducting, higher melting point metals.

By my invention I make the enclosure for the fuse link gas-tight in a simple, inexpensive manner which will be made clear by reference to the drawing.

In the single sheet of drawings Figure 1 shows a vertical section of a plug fuse embodying my invention. Fig. 2 shows a washer embodying my invention and such as is used in Fig. 1.

In the drawing in Fig. 1 is shown a fuse having a body member I of electric insulating material, such as porcelain, of a cup like form in the interior of which is mounted a fuse link 3 preil5 erably of one of the better conducting metals. Contact terminals 5 and I are mounted on the body and the ends of the fuse link are electrically connected to the contacts as by solder 9. A sealing compound or cement H is used to render gas-tight any openings in the walls of the bo member necessary to obtain the electrical connections between the link and the contact terminals.

A closure member I3 is secured to the open end of the cup-like body by means of a cap i5 spun over a shoulder IT on the body. The closure member may be of mica or other suitable transparent material if desired. A sealing washer, IQ, of annular shape is interposed between the end of body member and the closure member and there held in place by the cap. This washer is of yielding material and substantially impervious to gas and the securing of the cap to the body member is done in such manner that the Washer 35.15 somewhat compressed. This compensates for any slight irregularities in the surfaces of the end of the body member and the closure member and prevents the escape of vapors generated within the body cavity. The fact that the washer 4ois soft and held in compression prevents the escape of vapors from the body cavity at all operating temperatures. When a current is passed through a fuse its temperature rises and due to unequal expansion of parts the space between the closure member and the body member may vary. By making my washer yleldable and holding it compressed any such variation is compensated for and the escape of vapor prevented.

I may make the washer of a relatively soft paper thoroughly impregnated with paraflin oil to render it impervious to the passage of vapors through the pores thereof. This also prevents its absorbing moisture which might render the cap electrically alive should the washer contact any live part of the fuse link or connecting conducting parts.

It will be seen from the above description of my invention that the washer is designed to render the enclosure gas-tight by eifectively 5o blocking the passageway through which, otherwise, vapors might escape.

It will be obvious that a wide range of choice as to the material to be used for the sealing washer, as to the method by which it is retained as in place, is possible without departing from the pirit oi my invention.

What I claim is:

1. A plug fuse having a cup-like body member of moulded insulating material of substantially unchanging dimensions capable of withstanding the sudden and high internal pressures produced on blowing of the fuse, said body having a central contact extending through an opening in the base and another contact extending through an opening in the side thereof, both openings being sealed to prevent the exit of vapor therethrough, a closure member consisting of a thin sheet of transparent insulation material also capable, when positioned, of withstanding such pressures, a fuse link between saidcontacts,asealing member consisting of a slightlycompressibleinsulationmaterial substantially impervious to the flow of vapor therethrough placed on the open end of the cuplike body and beneath the closure member and means consisting of a ring-like member to clamp the closure member and the sealing member to the body to provide a substantially gas-tight enclosure.

2. A plug fuse having a cup-like body of moulded insulating material of substantially unchang ing dimensions capable of withstanding the sudden and high internal pressures produced on the blowing of the fuse, said body having a central contact extending through an opening in the base and another contact extending through an 7 opening in the side thereof, both openings being sealed to prevent the exit of vapor therethrough, a closure member consisting of a thin sheet of slightly flexible transparent insulating material also capable, when positioned, of withstanding such pressures, a fuse link of relatively high melting point between said contacts, a sealing member consisting of a slightly compressible insulation material substantially impervious to the flow of vapor therethrough placed on the open end of the cup-like body and beneath the closure memher and means consisting of a ring-like member to clamp the closure member and the sealing member to the body to provide a substantially gas-tight enclosure, whereby the pressure of the vapors produced on blowing of the fuse may effect a quick extinction of the arc.

3. A plug fuse having a cup-like body of insulating material, said body having a central contact extending through an opening in the base and another contact extending through an opening in the side thereof, both openings being sealed to prevent the exit of vapor therethrough, a closure member consisting of a thin sheet oi? insulating material, a fuse link between said contacts, a sealing member consisting of a thin annular disc of slightly compressible insulation material substantially impervious to the flow of vapor therethrough placed on the open end of the cup-like body and beneath the closure memher, and means to clamp the closure and sealing members to the body member to provide a substantially gas-tight chamber for said fuse link.

FRANK H. BUSSMANN. 

